Leap Day & Why We Need It

Time to leap for joy; it’s Leap Day. It’s the extra 24 hours we get every four years on February 29th.

The idea of adding an additional day to the calendar every four years started with Julius Caesar, and its main purpose is to keep the spring equinox at the same time each year.

“The main purpose was to help the farmers out. They wanted the seasons to be at the same time in the calendar year,” said Simon Schuler, Assistant Professor of Physics and Astronomy at The University of Tampa.

Without Leap Day, the calendar would be off by 24 days each 100 years. It has to do with how long it takes Earth to travel around the sun.

“We have leap year because our calendar does not match exactly with Earth’s orbital period of the earth around the sun,” Schuler explains.

The calendar year is 365 days, but it actually takes the earth 365 days, 5 hours, 48 minutes and 45 seconds. That is almost an extra quarter of a day each year. To account for that time, we add February 29th to the calendar every four years.

The current leap year strategy, adopted in 1582, calls for the extra day to be added to years divisible by four. For instance, 2016 divided by four is exactly 504, so 2016 is a leap year.

The strategy has to be amended slightly because the extra time around the sun is not quite a quarter of day. Therefore, leap year is skipped three times every 400 years.

The skipped years are century years that are not divisible by 400. That means we had a leap year in 2000 because if you divide 2000 by 400, the answer is exactly 5. We will skip the leap years in 2100, 2200 and 2300, but the leap day will be added again in 2400.

Despite all these rules, the current calendar is still 27 seconds too long, so it will be off by a full day every 3,236 years.